CHOICE¶

The ASN.1 CHOICE type represents a union-like data structure where only one of the possible fields can be set at a time.

Example ASN.1 definition:

MyChoice ::= CHOICE {
    intValue   INTEGER,
    strValue   UTF8String,
    boolValue  BOOLEAN
}

generates the following Python class:

class MyChoice(_Asn1Type):
    intValue: int
    strValue: str
    boolValue: bool

    class PRESENT(enum.IntEnum):
        PR_NOTHING = 0
        PR_intValue = 1
        PR_strValue = 2
        PR_boolValue = 3

    @property
    def present(self) -> PRESENT: ...
    def __init__(
        self,
        intValue: int = ...,
        strValue: str = ...,
        boolValue: bool = ...,
    ) -> None: ...

In Python, the generated class for a CHOICE behaves similarly to a C union:

Only one field can hold a value at any time.
Setting a field automatically clears all other fields (sets them to None).
Accessing a field that is not currently set returns None instead of raising an exception.
The currently selected field is tracked via a special enumeration called PRESENT, accessible via the present attribute.
An instance of a CHOICE may be created empty, representing an invalid or unset state.

Conceptual Representation¶

class _Asn1ChoiceType

Represents an ASN.1 CHOICE type.

present: Holds the current active field indicator as a member of PRESENT.

__init__(self, /, **members: Any) → None

Initializes the CHOICE instance.

Only one field may be set via keyword arguments.
If multiple fields are provided or none, the object may represent an invalid or unset state.
Setting a field clears all others.

<field_name>

Access or set the value of the named field.

Setting a field clears other fields.
Getting an unset field returns None.